In recent years, as recording has been conducted at ever greater density in information recording devices such as magnetic disks, typified by hard disks, there has been demand for a change from longitudinal magnetic recording systems to perpendicular magnetic recording systems. That is, it has been pointed out that in longitudinal magnetic recording systems, since the magnetic zone is readily rotated by heat at about room temperature, it becomes impossible to write and data that have been written tend to be lost as the recording density increases. This phenomenon is known as the problem of thermal fluctuation and is becoming an ever greater impediment in longitudinal magnetic recording methods. Accordingly, in response to the problem of thermal fluctuation in longitudinal magnetic recording methods, there has been an active research of the practical use of perpendicular magnetic recording methods in recent years.
Known film structures employed in perpendicular magnetic recording methods include a single-layer film formed over a perpendicular magnetic recording layer on a nonmagnetic substrate, a double-layer film obtained by sequentially stacking a soft magnetic layer and a magnetic recording layer on a nonmagnetic substrate, and a three-layer film obtained by sequentially stacking a hard magnetic layer, a soft magnetic layer, and a magnetic recording layer on a nonmagnetic substrate. Of these, the two-layer film and three-layer film are better suited to achieving high recording densities and the maintenance of a stable magnetic moment than the single-layer film, leading to substantial development focusing on practical use in recent years. The improvement of the characteristics of such multilayer film perpendicular magnetic recording media requires film formation with a high-temperature film forming unit such as a high-temperature sputtering device and high-temperature treatment following film formation.
When employing a glass substrate affording good processing properties and reliability as the substrate of various information recording media, including the magnetic recording media employed in the above-mentioned perpendicular magnetic recording method, it is necessary to solve problems such as the following.
The glass substrate for information recording media is subjected to precision polishing such as lapping and polishing to impart extremely high flatness and surface smoothness. However, there is a problem in that since the substrate is exposed to elevated temperatures in the course of forming a film serving as the information recording layer, the glass softens and deforms unless the thermal resistance of the substrate is high, precluding use as an information recording medium. Thus, there is a need for a glass material with high thermal resistance that does not deform when exposed to elevated temperatures.
This high thermal resistance is an important characteristic for ensuring flatness in a static state. Flatness is also demanded in high-speed rotation since the reading and writing of information is conducted with the information recording medium being rotated at high speed. Thus, a substrate that does not yield at high-speed rotation, that is, a substrate of a material with a high Young's modulus, is required.
In the above-mentioned film-forming step, the glass substrate is held and conveyed into and out of a high-temperature film-forming device. In the course of this conveyance, a substrate that has been heated to an elevated temperature is rapidly cooled, subjecting the glass substrate, particular the portion being held, to substantial stress due to thermal contraction. Similarly, there is also a problem in that the glass substrate is also sometimes subjected to substantial thermal shock during high-temperature heat processing following film formation, with the substrate being damaged by this shock. Thus, there is a need for a high-strength glass substrate capable of adequately withstanding thermal shock.
Further, an information recording medium such as that mentioned above rotates at an extremely high speed of several thousand rpm during operation. Thus, there is a strong need to increasing the strength of the glass substrate to prevent damage during high-speed rotation.
An example of a glass employable as substrate in an information recording medium is the chemically reinforceable alumina silicate glass disclosed in Patent Reference 1 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-72238). However, from 8.5 to 15.5 mol % of Na2O is incorporated into the glass described in Patent Reference 1 to enhance the glass melt property and increase ion-exchange efficiency for chemical reinforcement. Na2O has the effect of decreasing the Young's modulus of the glass. Thus, the glass described in Patent Reference 1 has a low Young's modulus and substrates produced from the glass have poor flatness when rotated at high speed. Nor is application to information recording media employed in perpendicular magnetic recording systems suggested in any way in Patent Reference 1.
Accordingly, the present invention has for its object to provide glass having both high thermal resistance and high strength, a substrate for use in information recording media comprised of this glass, and an information recording medium employing such a substrate.